1. Field of the Invention
The present invention relates generally to temperature compensated crystal oscillators, and more particularly to a miniaturized temperature compensated crystal oscillator, in which two cavities forming mounting structures for a crystal vibrating chip and an IC chip are arranged not to be vertically overlapped, thus allowing a separating layer between the two cavities to be removed.
2. Description of the Prior Art
Generally, crystal oscillators using crystal vibrating chips are essential parts to generate oscillation frequencies for controlling transmission and reception of signals between mobile communication terminals. The crystal oscillators have excellent frequency stability compared with other oscillators. However, the crystal vibrating chips are problematic in that their oscillation frequencies are varied due to ambient temperature, In order to solve the problem, the crystal oscillators have additional parts for compensating the frequency variation due to the temperature sensitivity of the crystal vibrating chips. Such oscillators are each so-called temperature compensated crystal oscillator (TCXO). The temperature compensated crystal oscillators are classified into two types according to the realization method of a temperature compensation circuit; one-chip type using an integrated circuit (IC) chip and a discrete type in which various parts such as a piezoelectric oscillating element, an integrated circuit, capacitors, inductors and resistors, are mounted. Hereinafter, various structures of temperature compensated crystal oscillators proposed in the prior art are described.
FIGS. 1a and 1b are a side sectional view and a plane view of a conventional discrete type TCXO 10, respectively. As shown in FIGS. 1a and 1b, the discrete type TCXO 10 has a structure in which a crystal oscillating unit 13 containing a crystal vibrating chip is arranged on the upper surface of a printed circuit layer (PCB) 11, and a plurality of parts 15 for a temperature compensation circuit are arranged at both side portions of the crystal oscillating unit 13 The term xe2x80x9ccrystal oscillating unitxe2x80x9d represents a surface mounted device type part constructed by packaging the crystal vibrating chip. The temperature compensation parts 15 generally occupy an area approximately 2 to 3 times as large as the crystal oscillating unit 13 with a size of 5.0xc3x973.2 mm2 or 4.7xc3x972.9 mm2. Further, the printed circuit board 11 used in the TCXO 10 requires an area much larger than the crystal oscillating unit 13, such that a final product installed with the crystal oscillating unit 13 is also increased in its size (for example, larger than 7.0xc3x975.2 mm2).
As described above, the difficulty in the miniaturization of discrete type TCXO restricts the employment of the TCXO as parts of mobile communication terminals. On the other hand, the one-chip type TCXO is advantageous in that the final product can be miniaturized by the use of an IC chip in which a plurality of parts such as a temperature compensation circuit and etc. are integrated, although its phase noise characteristics are somewhat bad in comparison with the discrete type TCXO. As a result, one-chip type TCXO is widely used recently.
FIGS. 2a and 2b are side sectional views of two different conventional one-chip type TCXOs. Referring to FIGS. 2a and 2b, one-chip type TCXOs 20 or 20xe2x80x2 are comprised of a layered structure 21 or 21xe2x80x2 having an IC chip 27 or 27xe2x80x2 in which a plurality of parts are integrated.
Especially, the temperature compensated crystal oscillator (TCXO) 20 of FIG. 2a is comprised of the layered structure 21 in which first to fourth layers 21a to 21d are stacked in turn and different cavities are formed in the third and fourth layers 21c and 21d, respectively. The layered structure 21 includes the IC chip 27 inserted into the cavity formed in the third layer 21c, wherein the IC chip 27 is bonded to connection pads 28 on the second layer 21b. Further, a crystal vibrating chip 23a is inserted into the cavity, which is formed in the fourth layer 21d and has an opening larger than the cavity of the third layer 21c. Finally, a metal cover 25 is mounted on the upper surface of the fourth layer 21d. 
The one-chip type TCXO 20 shown in FIG. 2a is advantageous in the miniaturization aspect. However, if the crystal vibrating chip 23a is damaged in the mounting process, the IC chip 27 must be also discarded due to the difficulty in the separation of the crystal vibrating chip 23a. That is, when the crystal vibrating chip 23a is inserted after the IC chip 27, most of the damage may occur in the process of inserting the crystal vibrating chip 23a. However, according to the process due to the structure, the crystal vibrating chip 23a is arranged after the IC chip 27 is mounted. Accordingly, even if damage of the crystal vibrating chip 23a is detected, it is difficult to reuse the already mounted IC chip 27 which is comparatively expensive, Therefore, this structure of FIG. 2a unnecessarily increases costs in the process for producing high quality products. Further, this structure is problematic in that the performance of the TCXO 20 is decreased due to mutual electromagnetic interference, because the TCXO 20 does not have shielding means such as a layer for blocking electrical influence between the crystal vibrating chip 23a and the IC chip 27, and so both of them directly influence each other.
In order to solve the problems, the surface mounted device-type crystal oscillator 20xe2x80x2 of FIG. 2b is proposed, wherein a crystal oscillating unit 23xe2x80x2 is mounted on top. As already explained in FIG. 1a, the term xe2x80x9ccrystal oscillating unitxe2x80x9d represents a surface mounted device type part constructed by packaging the crystal vibrating chip. The TCXO 20xe2x80x2 is constructed such that a cavity is formed in a layer 21cxe2x80x2, and an IC chip 27xe2x80x2 is mounted on connection pads 28xe2x80x2 formed on the upper surface of a layer 21bxe2x80x2 which forms the lower surface of the cavity using a flip chip bonding method, and finally the crystal oscillating unit 23xe2x80x2 is mounted on the upper surface of the top layer 21cxe2x80x2, In the surface mounted device-type crystal oscillator 20xe2x80x2, because the crystal vibrating chip is contained inside the crystal oscillating unit 23xe2x80x2, the damage occurring in the inserting process of the crystal vibrating chip can be prevented, and further electromagnetic interference between the IC chip 27xe2x80x2 and the crystal oscillator 23xe2x80x2 can be effectively shielded by the package surrounding the crystal vibrating chip, thus maintaining the performance of good quality of the TCXO 20xe2x80x2.
In order to basically solve the problems occurring in the TCXO 20 of FIG. 2a compared with the crystal oscillator of FIG. 2b, another structure of a temperature compensated crystal oscillator 30 of FIG. 3 is proposed. Referring to FIG. 3, the temperature compensated crystal oscillator 30 has a TCXO structure in which an additional cavity is formed in a lower layer 31b to insert an IC chip 37 into the cavity. Referring to FIGS. 3a and 3b, the TCXO 30 is comprised of upper layer regions 31d and 31e in which a cavity for mounting a crystal vibrating chip 33 is formed, and lower layer regions 31a and 31b in which a cavity for mounting the IC chip 37 is formed. In this case, the two cavities are separated by an additional layer 31c. Here, the upper layer regions 31d and 3e are elements corresponding to the surface mounted device-type crystal oscillating unit 23xe2x80x2 shown in FIG. 2b. 
According to the TCXO 30, the mounting space of the IC chip 37 is separated from that of the crystal vibrating chip 33 by the layer 31c arranged between the two cavities, such that additional mounting spaces for the IC chip 37 and the crystal vibrating chip 33 can be prepared. Therefore, the problems of the crystal oscillator of FIG. 2a can be basically solved by varying the layer structure, contrary to the crystal oscillator of FIG. 2b. In other words, the mounting spaces for the crystal vibrating chip 33 and the IC chip 37 are vertically separated from each other. Accordingly, even if the crystal vibrating chip 33 is damaged in the inserting process, there is no need to discard the IC chip 37. Further, because the crystal vibrating chip 33 and the IC chip 37 are separated by the layer 31c, mutual electromagnetic interference therebetween can be effectively blocked.
However, the temperature compensated crystal oscillator having the above structure is problematic in that it is difficult to mount the crystal oscillator to mobile communication terminals due to its increased height. Further, as slim mobile communication terminals have been popularized recently, it is gradually required to miniaturize the terminals through the reduction of their heights rather than their areas. Accordingly, in this technical field, a new TCXO structure for realizing the miniaturization through the reduction of height of each mobile communication terminal is required, while maintaining the advantages of the TCXO structure of FIG. 3 suitable for production of good products.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a temperature compensated crystal oscillator, in which two cavities for accommodating a crystal vibrating chip and an IC chip are each opened at its top and bottom, and arranged not to be vertically overlapped so as to remove a layer for separating the crystal vibrating chip and the IC chip, thereby reducing the height of a final product.
In order to accomplish the above object, the present invention provides a temperature compensated crystal oscillator, comprising a first layered structure comprised of at least one layer and provided with a cavity formed therein; a second layered structure arranged on the upper surface of the first layered structure, comprised of at least one layer, and provided with a cavity formed in a region not overlapped with the cavity of the first layered structure; an IC chip inserted into the cavity of the first layered structure; a crystal vibrating chip inserted into the cavity of the second layered structure; a resin mold portion formed by charging resin into the cavity of the first layered structure accommodating the IC chip to make its bottom surface level with the bottom surface of the first layered structure; and a metal cover arranged on the upper surface of the second layered structure for covering the cavity formed of the second layered structure.
According to a preferred embodiment, the first layered structure can be comprised of a plurality of layers, wherein an integrated cavity can be formed through the layers. Through this structure, the cavity having a sufficient height for mounting the IC chip can be formed, even though thin layers are used.
Moreover, it is preferable to provide a portion of the lower surface of the second layered structure as the upper surface of the cavity formed in the first layer structure, arrange conduction pads for flip chip bonding in the portion, and insert the IC chip into the cavity formed in the first layered structure, such that the IC chip is connected to the conduction pads using a flip chip bonding manner,
In the preferred embodiment of the present invention, the second layered structure is comprised of a first layer and second layer placed on the upper surface of the first layer. In this case, a cavity formed in the second layer has an opening larger than at least the crystal vibrating chip, and a cavity formed in the first layer has an opening smaller than at least the crystal vibrating chip, such that the cavities formed in the first and second layers can be formed as an integrated cavity having a stepped structure.
At this time, the crystal vibrating chip is arranged such that its one end is connected to the upper portion of a side wall of the cavity formed in the first layer, and its other end is contacted with the upper portion of an opposite side wall, such that the crystal vibrating chip can oscillate by receiving signals through a signal path formed at the upper portion of the side wall connected to its one end.
In another preferred embodiment of this invention, the first layered structure has a connection bump made of a conductor on its upper surface provided as the lower surface of the cavity formed in the second layered structure, and the crystal vibrating chip is inserted into the cavity formed in the second layered structure, such that crystal vibrating chip can oscillate by receiving signals through the connection bump and the signal path, with the crystal vibrating chip being fixed to the connection bump.
Moreover, the cavities formed in the first and second layered structures can be each constructed in the shape of a rectangle according to the shapes of IC chip and the crystal vibrating chip to be respectively inserted into the cavities.